Many of the light sources commonly used in fiber optic applications, particularly with single mode fibers, produce highly-polarized light. This is particularly true of lasers. However, in many situations unpolarized light is desired. For example, some light detection systems have a sensitivity that is dependent, in part, on the polarization of the light they are detecting. Alternatively, if a polarizing fiber, such as that described in commonly-assigned U.S. patent application Ser. No. 381,038, filed Jul. 17, 1989 is to be used with a polarized source, the amount of light emerging from the fiber will be related to the orientation of the polarization of the light entering with respect to the birefringence axis of the fiber. Thus measurements of the light intensity emerging from a system utilizing polarizing fiber must take into account such orientation. If the light entering the polarizing fiber is unpolarized, however, clearly such orientation is not a factor.
In order to appreciate the prior art depolarization systems, it must be understood that unpolarized light is not light lacking polarization. Rather it is light in which the instantaneous polarization state changes extremely rapidly so that there is no net preference for any one polarization state. One method that has been used previously for producing effectively unpolarized light is to split a polarized beam into a plurality of subbeams and then recombine them so as to produce a varying pattern of polarization states across the face of a detector. Because this involves a spatial average across a comparatively large area, it is not generally useful with single mode optical fibers.
A second approach provides a time average rather than a spatial average. This is done by varying the birefringence of an optical fiber through which the light is travelling at a rate greater than the response time of a detector to be used. While such an approach is clearly useful with optical fiber, it requires the additional complexity of an active system. A passive depolarizer would be preferable.
U.S. Pat. No. 4,968,112 (Lovely et al.) utilizes a lithium niobate waveguide. Such a waveguide will retard light having one polarization state relative to light having a different polarization state. The waveguide is chosen such that it is long enough that the retardation is sufficient to destroy the phase coherence of the two states. A problem with such a system is that if the coherence time of the light source is long, the required waveguide will be excessively long.